Micro pick up array with compliant contact

ABSTRACT

Micro pick up arrays for transferring micro devices from a carrier substrate are disclosed. In an embodiment, a micro pick up array includes a compliant contact for delivering an operating voltage from a voltage source to an array of electrostatic transfer heads. In an embodiment, the compliant contact is moveable relative to a base substrate of the micro pick up array.

BACKGROUND

1. Field

The present invention relates to micro devices. More particularlyembodiments of the present invention relate to micro pick up arrayshaving compliant contacts.

2. Background Information

Integration and packaging issues are one of the main obstacles for thecommercialization of micro devices such as radio frequency (RF)microelectromechanical systems (MEMS) microswitches, light-emittingdiode (LED) display systems, and MEMS or quartz-based oscillators.

Traditional technologies for transferring devices include, e.g.,“transfer printing”, which involves using a transfer wafer to pick up anarray of devices from a donor wafer. The array of devices are thenbonded to a receiving wafer before removing the transfer wafer. Sometransfer printing process variations have been developed to selectivelybond and de-bond a device during the transfer process. In bothtraditional and variations of the transfer printing technologies, thetransfer wafer is de-bonded from a device after bonding the device tothe receiving wafer. In addition, the entire transfer wafer with thearray of devices is involved in the transfer process.

More recently it has been proposed to transfer a semiconductor die froma host substrate to a target substrate using elastomeric stamps in whicha stamp surface adheres to a semiconductor die surface via van der Waalsforces.

SUMMARY OF THE INVENTION

Micro pick up arrays for transferring micro devices from a carriersubstrate are disclosed. In an embodiment, a micro pick up arrayincludes a base substrate having a via, a flexible membrane over thevia, and a plug supported by the flexible membrane and moveable relativeto the base substrate within the via. The flexible membrane may includea silicon layer and be deflectable such that the plug is moveable by notmore than 5 μm along an axis orthogonal to the flexible membrane,relative to the base substrate. A gap may separate the plug from thebase substrate. In an embodiment, a breakdown voltage of the gap may begreater than 100 volts at ambient pressure. For example, the gap mayseparate the plug from the base substrate by more than 10 μm to achievethe breakdown voltage.

In an embodiment, an array of electrostatic transfer heads may beelectrically coupled with the plug. The electrostatic transfer heads maybe deflectable into a cavity in the base substrate. Each electrostatictransfer head may include a mesa structure with an electrode surfacecovered by a dielectric layer. Each electrostatic transfer head may alsoinclude a second electrode surface covered by the dielectric layeradjacent the electrode surface. An electrode interconnect mayelectrically couple the electrode surface with the plug. Likewise, asecond electrode interconnect may electrically couple the secondelectrode surface with a second plug. For example, the electrodeinterconnect may couple with a topside contact on the plug. The topsidecontact may contact the plug over a contact area that is coplanar with atopside plug area and is less than two-thirds of the topside plug area.A contact pad may be on the plug opposite the topside contact and may beelectrically coupled with the topside contact through the plug. Anelectrical resistance across the plug between the contact pad andtopside contact may be in a range between 1 and 100 kiloohms.

In an embodiment, a method of forming a micro pick up array includesetching a top silicon layer of a silicon-on-insulator (SOI) stack toform an array of electrodes and etching through a bulk silicon substrateof the SOI stack to a buried oxide layer of the SOI stack to form a gapseparating a plug and a base substrate of the bulk silicon substrate.The plug may be moveable relative to the base substrate. The basesubstrate may also be etched to form one or more cavities directlyunderneath the array of electrodes such that one or more electrodes isdeflectable into the one or more cavities. The method of forming themicro pick up array may also include etching the top silicon layer toform an electrode interconnect, forming a dielectric layer over thearray of electrodes, and forming a topside contact on the bulk siliconsubstrate. Forming the dielectric layer may include thermal oxidation ofthe array of electrodes. Alternatively, forming the dielectric layer mayinclude blanket depositing the dielectric layer using atomic layerdeposition or depositing the dielectric layer using chemical vapordeposition. The method of forming the micro pick up array may alsoinclude etching through the dielectric layer, the electrodeinterconnect, and the buried oxide layer to expose the plug of the bulksilicon substrate. The topside contact may be formed on the exposed areaof the plug. The topside contact may be electrically coupled with thearray of electrodes through the electrode interconnect. The method offorming the micro pick up array may also include etching through abackside oxide layer of the SOI stack to expose the plug of the bulksilicon substrate and forming a contact pad on the plug of the bulksilicon substrate opposite the topside contact. The contact pad may beelectrically coupled with the topside contact through the plug.

In an embodiment, a system includes a transfer head assembly and a micropick up array. The transfer head assembly may include one or moreoperating voltage contacts and a clamping voltage contact. The micropick up array may include a base substrate, one or more compliantcontacts formed through the base substrate, and an array ofelectrostatic transfer heads on a frontside of the micro pick up arrayelectrically coupled with the one or more compliant contact. The one ormore operating voltage contacts may be alignable with the one or morecompliant contacts and the clamping voltage contact may be alignablewith a backside of the micro pick up array opposite the array ofelectrostatic transfer heads. Accordingly, when a clamping voltage isapplied to the clamping voltage contact the micro pick up array isretained against the transfer head assembly and the plug moves relativeto the base substrate.

In an embodiment, the micro pick up array may also include a via in thebase substrate, a flexible membrane over the via, and a plug supportedwithin the via by the flexible membrane. A gap may separate the plugfrom the base substrate and the plug may be movable relative to the basesubstrate. The array of electrostatic transfer heads may be electricallycoupled with the plug. Furthermore, each electrostatic transfer head mayinclude a mesa structure having an electrode surface, and a dielectriclayer covering the electrode surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustration of a transfer head assemblyholding a micro pick up array with a compliant contact in accordancewith an embodiment of the invention.

FIG. 2A is a plan view illustration of a micro pick up array having anarray of monopolar electrostatic transfer heads in accordance with anembodiment of the invention.

FIG. 2B is a plan view illustration of a micro pick up array having anarray of bipolar electrostatic transfer heads in accordance with anembodiment of the invention.

FIG. 3 is a combination cross-sectional side view illustration takenalong lines A-A, B-B, and C-C of FIG. 2B illustrating a micro pick uparray having an array of electrostatic transfer heads electricallycoupled with a compliant contact in accordance with an embodiment of theinvention.

FIG. 4A is a cross-sectional side view illustration taken along aportion of line B-B or C-C of FIG. 2B illustrating a compliant contactin accordance with an embodiment of the invention.

FIG. 4B is a cross-sectional side view illustration taken along aportion of line B-B or C-C of FIG. 2B illustrating a compliant contactwith a dielectric-filled gap in accordance with an embodiment of theinvention.

FIG. 5 is a perspective view illustration of a topside portion of amicro pick up array having a compliant contact in accordance with anembodiment of the invention.

FIG. 6A is a cross-sectional side view illustration of a moveableportion of a micro pick up array having a compliant contact supported bya flexible membrane in accordance with an embodiment of the invention.

FIG. 6B is a cross-sectional side view illustration of a moveableportion of a micro pick up array having a load applied to a compliantcontact supported by a flexible membrane in opposition to a clampingforce applied to a clamping area of the micro pick up array inaccordance with an embodiment of the invention.

FIGS. 7-24 illustrate a method of forming a micro pick up array havingan array of electrostatic transfer heads electrically coupled with acompliant contact in accordance with an embodiment of the invention.

FIG. 25 is a cross-sectional side view illustration of a system having amicro pick up array and a transfer head assembly in accordance with anembodiment of the invention.

FIG. 26 is a schematic top view illustration of contacts between a micropick up array and a transfer head assembly in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention describe apparatuses and methodsfor transferring a micro device or an array of micro devices. Forexample, the micro device or array of micro devices may be any of themicro LED device or micro chip structures illustrated and described inrelated U.S. patent application Ser. Nos. 13/372,222, 13/436,260,13/458,932, and 13/711,554. While some embodiments of the presentinvention are described with specific regard to micro LED devices, theembodiments of the invention are not so limited and certain embodimentsmay also be applicable to other micro LED devices and micro devices suchas diodes, transistors, integrated circuit (IC) chips, and MEMS.

In various embodiments, description is made with reference to thefigures. However, certain embodiments may be practiced without one ormore of these specific details, or in combination with other knownmethods and configurations. In the following description, numerousspecific details are set forth, such as specific configurations,dimensions, and processes, in order to provide a thorough understandingof the present invention. In other instances, well-known processes andmanufacturing techniques have not been described in particular detail inorder to not unnecessarily obscure the present invention. Referencethroughout this specification to “one embodiment,” “an embodiment”, orthe like, means that a particular feature, structure, configuration, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the invention. Thus, the appearances ofthe phrase “one embodiment,” “an embodiment”, or the like, in variousplaces throughout this specification are not necessarily referring tothe same embodiment of the invention. Furthermore, the particularfeatures, structures, configurations, or characteristics may be combinedin any suitable manner in one or more embodiments.

The terms “over”, “to”, “between”, and “on” as used herein may refer toa relative position of one layer or component with respect to otherlayers or components. One layer “over” or “on” another layer or bonded“to” another layer may be directly in contact with the other layer ormay have one or more intervening layers. One layer “between” layers maybe directly in contact with the layers or may have one or moreintervening layers.

Without being limited to a particular theory, embodiments of theinvention describe a micro pick up array supporting an array ofelectrostatic transfer heads which operate in accordance with principlesof electrostatic grippers, using the attraction of opposite charges topick up micro devices. In accordance with embodiments of the presentinvention, a pull-in voltage is applied to an electrostatic transferhead in order to generate a grip pressure on a micro device. The terms“micro” device or “micro” LED devices as used herein may refer to thedescriptive size of certain devices or structures in accordance withembodiments of the invention, such as on a scale of 1 to 100 μm.However, embodiments of the present invention are not necessarily solimited, and certain aspects of the embodiments may be applicable tolarger, and possibly smaller size scales. In an embodiment, a singlemicro device in an array of micro devices, and a single electrostatictransfer head in an array of electrostatic transfer heads both have amaximum dimension, e.g., a length or a width of a contact surface, of 1to 100 μm. In an embodiment, a pitch of an array of micro devices, and apitch of a corresponding array of electrostatic transfer heads is (1 to100 μm) by (1 to 100 μm). At these densities a 6 inch carrier substrate,for example, can accommodate approximately 165 million micro LED deviceswith a 10 μm by 10 μm pitch, or approximately 660 million micro LEDdevices with a 5 μm by 5 μm pitch. A transfer tool including a micropick up array and an array of electrostatic transfer heads matching aninteger multiple of the pitch of the corresponding array of micro LEDdevices can be used to pick up, transfer, and bond the array of microLED devices to a receiving substrate. In this manner, it is possible tointegrate and assemble micro LED devices into heterogeneously integratedsystems, including substrates of any size ranging from micro displays tolarge area displays, and at high transfer rates. For example, a 1 cm by1 cm array of electrostatic transfer heads can pick up, transfer, andbond more than 100,000 micro devices per transfer operation, with largerarrays of electrostatic transfer heads being capable of transferringmore micro devices.

In one aspect, embodiments of the invention describe a micro pick uparray having an array of electrostatic transfer heads and one or morecompliant contacts. The array of electrostatic transfer heads may besupported by a base substrate having a via. The compliant contact mayinclude a flexible membrane over the via and supporting a plug withinthe via. The back side of the plug can be physically coupled with atransfer head assembly that can be used to position the micro pick uparray including the array of electrostatic transfer heads. When aclamping force is applied to a clamping area on a backside of the micropick up array, an operating voltage contact of the transfer headassembly may apply an opposing reactive load to the plug, causing theflexible membrane to deflect. Deflection of the flexible membrane mayresult in the base substrate moving around the plug to produce relativemovement between the plug and the base substrate. Thus, while the micropick up array is secured to the transfer head assembly by the clampingforce, the reactive load creates compressive loading and pressurebetween the operating voltage contact of the transfer head assembly andthe plug, such that a uniform electrical contact is providedtherebetween.

In another aspect, embodiments of the invention describe a manner offorming a micro pick up array having an array of electrostatic transferheads and one or more compliant contacts from a commercially availablesilicon-on-insulator (SOI) stack. Embodiments of the invention describeforming portions of the micro pick up array, e.g., an array ofelectrodes, an electrode interconnect, and one or more compliantcontacts, etc., from the SOI stack using semiconductor devicefabrication processes.

In another aspect, embodiments of the invention describe applying avoltage through the compliant contacts to the electrostatic transferheads to create a gripping pressure between the array of electrostatictransfer heads and an array of micro devices. More specifically, anelectrostatic charge may be generated at the array of electrostatictransfer heads to grip the array of micro devices during transfer.Furthermore, the electrostatic charge may be maintained by a voltagedelivered to the electrostatic transfer heads through a plug of acompliant contact. Since the electrode circuit may operate underelectrostatic conditions during most of the transfer operation, the plugmay be considered to transfer an electrostatic voltage, rather than anelectrical current. Thus, the pick up and placement of micro devices maybe relatively insensitive to a response time of the electrode circuitand/or the plug. As a result, in an embodiment, an electrical resistanceacross the plug may be in a range higher than 1 to 1,000 ohms withoutcompromising pick up and placement.

Referring now to FIG. 1, a perspective view of a transfer head assemblyholding a micro pick up array with a compliant contact is illustrated inaccordance with an embodiment of the invention. Transfer head assembly102 may be a component of a larger system, such as a mass transfer toolused to transfer micro devices from a carrier substrate to a receivingsubstrate using micro pick up array 104. Transfer head assembly 102 mayretain micro pick up array 104 in numerous manners, including clips,vacuum ports, and by clamping one or more clamping areas on a backsidesurface of micro pick up array 104 with an electrostatic grippingpressure. For example, in an embodiment, transfer head assembly 102 mayinclude an electrostatic clamping contact that can receive anelectrostatic voltage from a voltage source. The clamping contact mayphysically appose a clamping pad or clamping area on a backside surfaceof micro pick up array 104. Thus, micro pick up array 104 may be grippedand retained against the transfer head assembly 102 by the clampingcontact.

In addition to delivering an electrostatic voltage to the clampingcontact on the transfer head assembly 102 to grip the micro pick uparray 104, the transfer head assembly 102 may deliver one or moreelectrostatic transfer head operation voltages to from voltage sources106, 206 to voltage interconnects 108 of micro pick up array 104.Voltage interconnects 108 may be compliant contacts. In addition tobeing compliant contacts, voltage interconnects 108 may also relayelectrostatic voltage through micro pick up array 104 into electrodeinterconnects 112 toward an array of electrostatic transfer heads 114.Thus, micro pick up array 104 may include compliant contacts that areboth compliant and able to transfer electrostatic voltage through micropick up array 104.

Referring now to FIG. 2A, a plan view illustration of a micro pick uparray having an array of monopolar electrostatic transfer heads isillustrated in accordance with an embodiment of the invention. The micropick up array 104 may include a plurality of electrostatic transferheads 114 formed in an array on a front side surface. Each electrostatictransfer head 114 may be electrically coupled with an electrodeinterconnect 112 running over the front side surface and placed inelectrical connection with a voltage interconnect 108. The voltageinterconnect 108 may include numerous structures, which are describedfurther below and allow for the transfer of voltage from a back sidesurface of the micro pick up array 104 to the front side surface. Forexample, in an embodiment, voltage interconnect 108 includes a compliantcontact having a plug and a flexible membrane. Thus, when micro pick uparray 104 is electrically coupled with voltage source 106, a voltage canbe transferred to electrode surface 202 on electrostatic transfer head114.

In the embodiment illustrated in FIG. 2A, the voltage interconnect 108on the left side of the illustration may be connected to voltage source106 denoted V_(A), and the voltage interconnect 108 on the right side ofthe illustration may be connected to a voltage source 206 denoted V_(B).Alternatively, the voltage interconnect 108 on the right side of theillustration may connect to the voltage source 106 denoted V_(A). Whereeach transfer head is operable as a monopolar transfer head, voltagesources 106 denoted V_(A) and 206 denoted V_(B) may simultaneously applythe same voltage so that each electrode surface 202 has the samevoltage. However, as described below, this arrangement for monopolarelectrostatic transfer heads 114 is not limiting.

Referring now to FIG. 2B, a plan view illustration of a micro pick uparray 104 having an array of bipolar electrostatic transfer heads 114 isillustrated in accordance with an embodiment of the invention. As inFIG. 2A, each electrostatic transfer head 114 may be electricallycoupled with voltage interconnects 108 through electrode interconnects112. The voltage interconnects 108 may include a compliant contact, asin FIG. 2A. However, in the embodiment illustrated in FIG. 2B, eachelectrostatic transfer head 114 is bipolar, and includes electrodesurface 202 and second electrode surface 204. Thus, in an embodiment,the upper and lower electrode interconnects 112 in the illustration maybe connected to a voltage source 106 denoted V_(A), and the middleelectrode interconnect 112 in the illustration may be connected to asecond voltage source 206 denoted V_(B). Where each electrostatictransfer head 114 is operable as a bipolar transfer head, voltage source106 denoted V_(A) may simultaneously apply a voltage to electrodesurface 202 that is opposite to a voltage applied to second electrodesurface 204 by second voltage source 206 denoted V_(B). Thus, eachelectrostatic transfer head 114 may include a pair of oppositely chargedelectrodes, leading to enhanced gripping pressures on correspondingmicro devices. For example, gripping pressures between each bipolarelectrostatic transfer head 114 and a corresponding micro device can beabout 20 atm or higher.

The monopolar and bipolar electrostatic transfer head configurations maybe interchangeable in various embodiments of micro pick up array 104.Indeed, micro pick up array 104 may include alternative patterns for thearray of electrostatic transfer heads 114, electrode interconnects 112,etc., depending on the available space on transfer head assembly 102,the micro device pattern on the carrier substrate, the bonding patternon the receiving substrate, and other features incorporated in micropick up array 104. For example, micro pick up array 104 may optionallyinclude features such as flexible cantilever beams 210 that suspendelectrostatic transfer heads 114 over one or more cavities 212underneath the array of electrostatic transfer heads 114. Electrodeinterconnects 112 may be routed over or within flexible cantilever beams210 over cavities 212.

Although the description below is made in relation to a bipolarelectrode configuration, the description is also applicable to otherelectrode configurations, e.g., monopolar electrode configurations.Furthermore, although the description below is made in relation to micropick up array 104 incorporating cavities 212, such features are notrequired. The compliant contacts described below may be incorporatedinto a variety of micro pick up array designs and are not limited to thespecific micro pick up array embodiments described and illustratedherein.

Referring now to FIG. 3, a combination cross-sectional side viewillustration is taken along lines A-A, B-B, and C-C of FIG. 2Billustrating a micro pick up array having an array of electrostatictransfer heads electrically coupled with a pair of compliant contacts inaccordance with an embodiment of the invention. The combination views donot precisely represent the sizes or locations of the features of micropick up array 104, but rather, are intended to combine features into asingle view for ease of description. For example, while the combinationcross-sectional side view illustrations show voltage interconnect 108 ofFIG. 2B having plug 304, contact pad 306, and topside contact 307electrically connected with only one electrode surface 202 throughelectrode interconnect 112, it is clear from FIG. 2B and theaccompanying description that voltage interconnect 108 may beelectrically connected with several electrode surfaces 202 through oneor more electrode interconnects 112.

In an embodiment, the cross-section taken along line A-A corresponds toa portion of micro pick up array 104 that includes a bipolarelectrostatic transfer head 114. The bipolar electrostatic transfer head114 includes electrode surface 202 and second electrode surface 204,both over a top surface of mesa structures 311. A dielectric layer 312may cover electrode surface 202 and second electrode surface 204, andmay also cover a side surface of mesa structures 311 laterally betweenthe pair of mesa structures 311 for the pair of electrodes in a bipolarelectrostatic transfer head 114. Thus, the top surface of dielectriclayer 312 over electrode surface 202 and second electrode surface 204 isoffset from, e.g., above electrode interconnect 112, and provides araised contact point for pressing against a micro device on a carriersubstrate or receiving substrate.

In an embodiment, dielectric layer 312 and buried oxide layer 314surround and separate mesa structures 311 and electrode interconnect 112of individual electrode circuits from each other and from other portionsof micro pick up array 104 to isolate a desired pathway between voltagesources 106, 206 and respective electrostatic transfer heads 114, and toprevent shorting between electrode surfaces 202, 204, electrodeinterconnects 112, and voltage interconnects 108 that are maintained atdifferent electrical potentials.

The embodiment illustrated in FIG. 3 includes electrostatic transferhead 114 supported above cavity 212 by a flexible cantilever beam 210,such that electrostatic transfer head 114 is deflectable into cavity212. In other embodiments, cavity 212 is not present.

Referring now to FIG. 4A, a cross-sectional side view illustration takenalong a portion of line B-B or C-C of FIG. 2B illustrates a compliantcontact in accordance with an embodiment of the invention. Morespecifically, the cross-section taken along lines B-B or C-C correspondsto a portion of micro pick up array 104 that includes voltageinterconnect 108 having a compliant contact. Thus, voltage interconnect108 transfers voltage from a voltage source 106 or 206 to electrodeinterconnect 112, but may also be moveable relative to other portions ofmicro pick up array 104, e.g., base substrate 214 or electrostatictransfer head 114. In an embodiment, base substrate 214 includes via 402extending from a backside surface of micro pick up array 104 to buriedoxide layer 314. Via 402 may have numerous cross-sectional shapes, forexample, via 402 may be cylindrical and have a circular cross-section.Alternatively, the cross-section of via 402 may be rectangular,rectangular with rounded corners, oval, etc.

In an embodiment, via 402 is partially filled by plug 304, which extendsthrough via 402 from buried oxide layer 314 and is laterally separatedfrom the surrounding base substrate 214 by gap 308. Plug 304 may beformed separately or simultaneously with via 402. For example, in anembodiment, plug 304 may be deposited onto a backside surface of buriedoxide layer 314 through via 402. In an alternative embodiment, gap 308may be formed by etching through a bulk silicon substrate, and thus,plug 304 is defined by the removal of material occupying gap 308.Regardless of the method used to form via 402 and plug 304, gap 308 maysurround the periphery of plug 304, resulting in plug 304 being coupledwith base substrate 214 by flexible membrane 310. As illustrated in FIG.4A, the width of flexible membrane 410 may be represented by the gap 308surrounding the periphery of plug 304. For example, where via 402 andplug 304 are circular, the width of flexible membrane 410 may be thedifference in the radii of the via 402 and plug 304.

Since gap 308 may extend around the periphery of plug 304, it mayprovide a dielectric barrier between plug 304 and base substrate 214.More particularly, gap 308 may prevent discharge from plug 304 to basesubstrate 214 when a voltage is applied to plug 304 from voltage source106 or 206. To function as a dielectric barrier, gap 308 may be shapedand sized depending on the operating voltage of micro pick up array 104.For example, in some embodiments, micro pick up array 104 operates withan electrostatic voltage of between about 100 to 150 volts appliedthrough contact pad 306 and plug 304 to electrostatic transfer heads114. Accordingly, gap 308 may be an air-filled space around plug 304with a breakdown voltage of at least 100 volts at ambient pressure. Inan embodiment, assuming that the breakdown voltage of air is about 327volts at standard atmospheric pressure across a gap distance of 7.5 μm,gap 308 distance may be maintained higher than about 10 μm to preventdischarge across gap 308. In an embodiment, the minimum distance acrossgap 308 may be between about 10 and 300 μm or more to prevent breakdownat normal operating conditions. More specifically, the minimum distanceacross gap 308 may be chosen to be about 20 μm.

Plug 304 may be concentrically located within via 402 such that gap 308is uniformly distributed around plug 304 periphery. Alternatively, plug304 may be configured within via 402 such that gap 308 distance betweenplug 304 and base substrate 214 varies. For example, via 402 may beshaped differently from plug 304, or plug 304 may be eccentricallylocated within via 402, such that gap 308 distance varies. Nonetheless,a minimum distance across the gap 308 may be controlled to achieve therequired breakdown voltage and to accommodate the operating voltagedelivered through plug 304.

Referring now to FIG. 4B, a cross-sectional side view illustration takenalong a portion of line B-B or C-C of FIG. 2B illustrates a compliantcontact with a dielectric-filled gap in accordance with an embodiment ofthe invention. In alternative embodiments, the breakdown voltage of thegap 308 may be controlled by introducing a suitable dielectric substanceinto gap 308. For example, gap 308 may be filled with a fluid thatdeforms under shear stress. For example, gap 308 may be filled with aliquid dielectric 406, such as a silicone oil, that does not impederelative movement between plug 304 and base substrate 214, but whichalso has a higher dielectric constant than air and allows for thedistance across gap 308 to be narrowed, as compared to gap 308 filledwith air, while still maintaining the requisite breakdown voltage of gap308. The gap can be filled with liquid dielectric 406 by, for example,dispensing liquid dielectric 406 into gap 308 using an air-powered fluiddispenser, a syringe, or another type of dispenser that can injectcontrolled volumes of fluid into small areas. Depending on the viscosityof liquid dielectric 406 that is inserted into gap 308, there may be aneed to retain liquid dielectric 406. For example, in the case wheresurface tension alone is unable to keep liquid dielectric 406 fromflowing out of gap 308, a seal 408 may be formed over or within gap 308to prevent liquid dielectric 406 from leaving gap 308. In an embodiment,seal 308 may include a flexible adhesive material, such as a siliconepolymer, deposited as a thin layer within gap 308 to bond with basesubstrate 214 and plug 304 while retaining liquid dielectric 406. Seal408 may be thin and flexible so as not to impede relative movementbetween plug 304 and base substrate 214.

In an alternative embodiment, non-liquid dielectrics, such as solid orgaseous dielectric materials may be introduced into and sealed withingap 308. For example, gap 308 may be at least partially filled with asolid dielectric including polymers such as acrylic, polyimide, orepoxies. The polymer dielectric may be introduced into gap 308 using anink-jetting process.

In FIGS. 4A-4B, flexible membrane 310 may permit relative movementbetween plug 304 and base substrate 214. In an embodiment, flexiblemembrane 310 may be sized to flex when opposing loads are applied toplug 304 and base substrate 214. The physical dimensions and materialproperties of top silicon layer 404 and gap 308 may be the leadingcontributors to the overall stiffness and flexibility of flexiblemembrane 310. In an embodiment, an overall thickness of flexiblemembrane 310 includes portions of top silicon layer 404, buried oxidelayer 314, and dielectric layer 312 that are located over via 402. In anembodiment, the width of flexible membrane 310 may be between about 10to 50 times the overall thickness of flexible membrane 310. For example,where the width of the flexible membrane is 10 times the overallthickness of flexible membrane 310, the thickness is about 5 μm while,as described above, flexible membrane 310 width may be about 50 μm.

Referring now to FIG. 5, a perspective view illustration of a topsideportion of a micro pick up array having a compliant contact isillustrated in accordance with an embodiment of the invention. In anembodiment, electrode interconnect 112 includes an electrode trace,wire, or other connector electrically connected with topside contact307. For example, electrode interconnect 112 may run over buried oxidelayer 314 and base substrate 214 from mesa structure 311 to topsidecontact 307. A path of electrode interconnect 112 may vary depending onthe topside geometry of micro pick up array 104, taking into accountfeatures such as flexible cantilever beams 210 supporting electrostatictransfer heads 114. Therefore, electrode interconnect 112 pattern mayinclude various bends, curves, etc. Furthermore, dielectric layer 312may cover electrode interconnect 112. In contrast, rather than beingcovered by dielectric layer 312, topside contact 307 may instead extendthrough dielectric layer 312, electrode interconnect 112, and buriedoxide layer 314, to a topside plug area 504.

Topside plug area 504 is represented with hidden lines to illustratethat it may be supported by flexible membrane 310 and under buried oxidelayer 314. Topside plug area 504 may correspond to a portion of plug 304that apposes buried oxide layer 314. Thus, topside contact 307 maycontact topside plug area 504 over contact area 506. Contact area 506may be proportionally less than topside plug area 504 because contactarea 506 may be no larger than plug 304 width and because minimizingcontact area 506 mitigates the risk of buried oxide layer 314delaminating from topside plug area 504. In an embodiment, contact area506 may be less than about half of topside plug area 504. For example,contact area 506 may have an effective diameter of between about 50 to100 μm while topside plug area 504 may have an effective diameter ofbetween about 300 to 500 μm. However, other contact area 506 and topsideplug area 504 dimensions may be used to similarly minimize the ratiobetween contact area 506 and topside plug area 504, and to provide astrong interface between topside plug area 504 and buried oxide layer314.

Topside contact 307 may also transfer voltage. In an embodiment, topsidecontact 307 provides an electrical pathway from plug 304 to electrodeinterconnect 112 through buried oxide layer 314 without considerablycompromising the function of flexible membrane 310. To provide thispathway, topside contact 307 may be formed from various conductivematerials, such as gold, NiCr, Cr, TiW, Ti, Al, alloys thereof orpolysilicon, that provide for electrical conductivity between plug 304and electrode interconnect 112.

As described above with regard to the structures shown along lines A-Aand B-B of FIG. 3, voltage interconnect 108 may include contact pad 306on a backside surface of plug 304. Contact pad 306 may be electricallycoupled with a corresponding operating voltage contact of transfer headassembly 102 to transfer voltage from voltage source 106 or 206. Thus,voltage may be delivered through contact pad 306 into plug 304, andtoward topside contact 307 on topside plug area 504. Topside contact 307may further be electrically coupled with electrode interconnect 112, andresultantly, voltage may be delivered from voltage source 106 or 206through plug 304 and electrode interconnect 112 to electrode surface202. Furthermore, voltage source 106 or 206 may transfer voltage tosecond electrode surface 204 of a bipolar electrostatic transfer head114 in a similar manner using corresponding structures shown along linesA-A and C-C of FIG. 3.

Operation of micro pick up array 104 may include the application andremoval of voltage to and from the array of electrostatic transfer heads114. For example, voltage may be applied to electrostatic transfer heads114 through plug 304 to grip micro devices and the voltage may beremoved from electrostatic transfer heads 114 to release micro devices.This application and removal may be accompanied by a spike in electricalcurrent as charge is generated or dissipated in the array ofelectrostatic transfer heads 114. However, during steady state operationof the array of electrostatic transfer heads 114, minimal or no currentis required to be delivered through plug 304 since the charge can bemaintained with minimal power draw from voltage source 106 or 206.Therefore, electrical resistance across plug 304 between contact pad 306and topside contact 307 may be less than about 25 kiloohms withoutdegrading the RC time constant of an electrode circuit to a point thatmicro pick up array 104 is unable to transfer micro devices in themanner described below. More specifically, since the pick up andplacement of micro devices occurs over relatively long periods of time,e.g., seconds, as compared to the response time of the electrodecircuit, e.g., microseconds, resistance across plug 304 may be increasedwithout disrupting the ability to pick up or place the micro devices.For example, electrical resistance across plug 304 between contact pad306 and topside contact 307 may be in a range higher than 1 to 1,000ohms. In an embodiment, electrical resistance across plug 304 may be inthe megaohm range without compromising the transfer of micro devices asdescribed in the following description. More specifically, in anembodiment, plug 304 has a nominal resistance value in a range of about1 to 100 kiloohms.

Referring now to FIG. 6A, a cross-sectional side view illustration of amoveable portion of a micro pick up array having a compliant contactsupported by a flexible membrane is illustrated in accordance with anembodiment of the invention. Prior to attaching micro pick up array 104to transfer head assembly 102, i.e., when no external loads are beingapplied to micro pick up array 104, flexible membrane 310 may havesufficient resilience to flatten across gap 308 and bring plug 304 intoalignment with base substrate 214 relative to axis 302.

Referring now to FIG. 6B, a cross-sectional side view illustration of amoveable portion of a micro pick up array having a load applied to acompliant contact supported by a flexible membrane in opposition to aclamping force applied to a clamping area of the micro pick up array isillustrated in accordance with an embodiment of the invention. Whenmicro pick up array 104 is clamped to transfer head assembly 102, e.g.,by applying an electrostatic clamping load 601 to pull a clamping areaover base substrate 214 toward a clamping contact of transfer headassembly 102, reactive load 602 may be applied to plug 304 by anoperating voltage contact of transfer head assembly 102. This reactiveload may be applied, for example, due to a mismatch in position betweena surface of the clamping contact and a surface of the operating voltagecontact. More specifically, the operating voltage contact may extendfurther from transfer head assembly 102 than the clamping contact.Accordingly, the operating voltage contact touches contact pad 306before the clamping contact touches the clamping area over basesubstrate 214 and flexible membrane 310 encounters a bending moment thatcauses it to deflect. This deflection permits plug 304, which floatswithin via 402, to move relative to base substrate 214. As flexiblemembrane 310 deflects and plug 304 moves, both base substrate 214 andplug 304 remain in contact with the clamping contact and operatingvoltage contact of transfer head assembly 102, respectively. Morespecifically, flexible membrane 310 accommodates relative movementbetween base substrate 214 and plug 304 to allow micro pick up array 104to be secured to transfer head assembly 102 while establishing anelectrical connection between plug 304 and voltage sources 106, 206.

The deflection of flexible membrane 310, and thus the movement of basesubstrate 214 relative to plug 304, depends on numerous characteristicsof the micro pick up array 104, and each of these characteristics may bemodifiable to adjust the degree of movement between base substrate 214and plug 304 that results from, e.g., various offsets between surfacesof a clamping contact and an operating voltage contact of the transferhead assembly 102. Without exhaustively listing these variables, some ofthe micro pick up array 104 characteristics that may be modified arewidth of flexible membrane 310 and stiffness of top silicon layer 404(FIG. 4). An example of the impact of just these two variables isprovided through a model in which top silicon layer 404 within flexiblemembrane 310 has a thickness of 5 μm. In a first instance, whereflexible membrane 310 is modeled with a gap 308 width of 50 μm and topsilicon layer 404 has a stiffness of 233 mN/μm, movement of basesubstrate 214 relative to plug 304 is estimated to be about 0.4 μm whena clamping load 601 and reactive load 602 correspond to a 300 MPapressure applied to plug 304. Alternatively, when the same pressure isapplied to a plug 304 with a flexible membrane 310 having a gap 308width of 100 μm and a top silicon layer 404 with a stiffness of 34mN/μm, movement of plug 304 relative to base substrate 214 is estimatedto be about 1.1 μm. In either of these alternatives, via 402 may have adiameter of about 2000 μm and a depth of about 600 μm. These estimatesshow not only that movement of plug 304 is affected by factors that areboth external and internal to micro pick up array 104, e.g., gap width(internal factor) and loading pressure (external factor), but alsoillustrates that these factors are controllable through micro pick uparray 104 design to tune movement of plug 304 relative to base substrate214 under the expected operating conditions of micro pick up array 104.

Referring now to FIG. 25, a cross-sectional side view of a system havinga micro pick up array and a transfer head assembly is shown inaccordance with an embodiment of the invention. Micro pick up array 104may be physically and electrically coupled with transfer head assembly102. More specifically, base substrate 214 of micro pick up array 104,or more particularly backside dielectric layer 1402 over base substrate214, may be physically secured to a clamping contact 2504 of transferhead assembly 102. Contact pad 306 of micro pick up array 104 may alsobe electrically coupled with an operating voltage contact 2502 oftransfer head assembly 102.

Transfer head assembly may include one or more clamping contact 2504. Inan embodiment, clamping contact 2504 is electrically coupled with aclamping voltage source 2506 to supply an electrostatic voltage toclamping contact 2504. Clamping contact 2504 may include a conductiveelectrode, optionally covered by a thin dielectric layer. Thus, byaligning the energized clamping contact 2504 with a backside of basesubstrate 214, an electrostatic voltage may be supplied to clampingcontact 2504 that exerts clamping load 601 on base substrate 214.Clamping load 601 may pull in on base substrate 214 to physically securemicro pick up array 104 to transfer head assembly 102.

Transfer head assembly may also include one or more operating voltagecontacts 2502. In an embodiment, an operating voltage contact 2502 isaligned with a contact pad 306 prior to securing micro pick up array 104to transfer head assembly 102. Operating voltage 2502 may include a bareconductor, such as a metallic pin. In accordance with embodiments of theinvention, as base substrate 214 is attracted toward clamping contact2504, operating voltage contacts 2502 and exerts a reactive load 602upon contact pad 306. Reactive load 602 may deflect flexible membrane310, causing plug 304 to move relative to base substrate 214 and createa residual compressive load between operating voltage contact 2502 andclamping pad 306. This residual compressive load may persist while micropick up array 104 is secured to transfer head assembly 102. Furthermore,the residual compressive load may result in a firm pressure between thecontacting surfaces that creates a uniform surface interface and arobust electrical contact. Therefore, the flexibility of flexiblemembrane 310 allows for an electrostatic voltage to be reliably suppliedfrom voltage sources 106, 206 through one or more operating voltagecontacts 2502 into one or more contact pads 306.

In accordance with some embodiments of the invention, the top contactsurfaces of the electrostatic transfer heads 114 protrude further awayfrom the micro pick up array than the surfaces adjacent the deflectedcompliant contacts. In this manner the deflected compliant contacts donot interfere with operation of the transfer head assembly. For example,in the exemplary embodiments described above the plug moves 0.4 μm-1.1μm relative to the base substrate when deflected. As will be describedin further detail below, the height of the electrostatic transfer headsmay be greater than the range of deflection of the compliant contacts.In an embodiment, the height of the mesa structures defining electrodesurfaces 202, 204 (see FIG. 12) rising above the silicon interconnects112 is greater than range of relative movement between the plug and basesubstrate.

FIG. 26 is a schematic top view illustration of contacts between a micropick up array and a transfer head assembly in accordance with anembodiment of the invention. In one embodiment, the contact area of theone or more clamping contacts 2504 on the transfer head assembly may belarger than the area 115 on the micro pick up array containing the arrayof transfer heads 114. Thus, the contact area of the clamping contact(s)2504 may be around the area 115 containing the array of transfer heads114. In this manner, the alignment and planarity across the array oftransfer heads 114 can be regulated by the alignment of the transferhead assembly. In such an embodiment, a plurality of compliant contacts,referenced by the plugs 304 in FIG. 26, are outside the periphery of theareas 2504, 115. In the particular embodiment illustrated, compliantcontacts are positioned on four sides of the area 115 including thearray of transfer heads 114.

Referring now to FIG. 7-24, a method of forming a micro pick up arrayhaving an array of electrostatic transfer heads electrically coupledwith one or more compliant contacts is illustrated in accordance with anembodiment of the invention. The processing sequence may begin with acommercially available SOI stack 702, as illustrated in FIG. 7. The SOIstack 702 may include bulk silicon substrate 704, top silicon layer 404,buried oxide layer 314 between bulk silicon substrate 704 and the topsilicon layer 404, and backside oxide layer 706. In an embodiment, bulksilicon substrate 704 is a silicon (100) handle wafer having a thicknessof 500 μm+/−50 μm, buried oxide layer 314 is 1 μm+/−0.1 μm thick, andtop silicon layer 404 is 7-20 μm+/−0.5 μm thick. The top silicon layer404 may also be doped to improve conductivity. For example, aphosphorous dopant concentration of approximately 10¹⁷ cm⁻³ yields aresistivity of less than 0.1 ohm-centimeter. In an embodiment, thebackside oxide layer 706 is a thermal oxide having a thickness up toabout 2 μm thick, which is the approximate upper limit for thermaloxidation of silicon.

Referring to FIG. 8, a mask layer 802 may be formed over the top siliconlayer 404. Mask layer 802 may be deposited, or alternatively thermallygrown from the top silicon layer 404. In an embodiment, mask layer 802is a thermally growth SiO₂ layer having a thickness of approximately 0.1μm. In an embodiment, where mask layer 802 is thermally growth SiO₂, themask layer 802 has a thickness which is significantly less than thethickness of buried oxide layer 314. This helps maintain structuralstability for the partially patterned SOI stack 702 during removal ofthe patterned mask layer 802.

Referring to FIG. 9, the mask layer 802 is then patterned to form anarray of islands 902 which will correspond to the mesa structures 311 ofelectrostatic transfer heads 114. In an embodiment, mask layer 802 is athermally grown SiO₂ layer, and islands 902 are formed by applying apositive photoresist, exposing, and removing undeveloped areas of thephotoresist with a potassium hydroxide (KOH) developer solution. Themask layer 802 is then dry etched, stopping on top silicon layer 404, toform islands 902 using a suitable technique such as ion milling, plasmaetching, reactive ion etching (RIE).

The array of islands 902 correspond to mesa structures 311 ofelectrostatic transfer heads 114 and are sized accordingly. In anembodiment, a length and a width of islands 902 correspond to electrodesurfaces 202, 204 of electrostatic transfer heads 114 that are betweenabout 1 to 100 μm. For example, an island 902 may have length and widthdimensions of 10 μm by 10 μm corresponding to an electrode surface 202having length and width dimensions of 10 μm by 10 μm, or a length andwidth dimensions of 2.5 μm by 2.5 μm corresponding to an electrodesurface 202 having length and width dimensions of 2.5 μm by 2.5 μm.However, these dimensions are exemplary, and other dimensions areenvisioned in accordance with embodiments of the invention. As a contactsurface of electrostatic transfer head 114 varies, e.g., between about 1and 100 μm in length and/or width, dimensions of islands 902 may bevaried accordingly. Islands 902 may be sized and located according towhether micro pick up array 104 includes monopolar or bipolarelectrodes. Thus, in the case of a monopolar design, only a singleisland 902 is required over each electrostatic transfer head 114. In theembodiment shown in FIG. 9, two islands 902 are placed over anelectrostatic transfer head 114, corresponding to a bipolar electrodedesign.

Referring to FIGS. 10-13, the mesa structures 311 and electrodeinterconnects 112 are patterned in a multi-part etching sequence. First,as illustrated in FIG. 10, the top silicon layer 404 between islands 902is etched through to form trench 1002. In an embodiment, this may beaccomplished using a thin patterned positive photoresist and DRIEetching through top silicon layer 404 to buried oxide layer 314. Thepatterned positive photoresist can be removed, resulting in thestructure illustrated in FIG. 10. Second, as illustrated in FIG. 11, thetop silicon layer 404 is partially etched, defining the mesa structures311 and the electrode interconnects 112. In an embodiment, this may beaccomplished with a thin patterned positive photoresist or with athermal oxide mask followed by DRIE etching, e.g., to removeapproximately 5 μm of a 7-10 μm thick top silicon layer 404 in a timedetch, resulting in the structure illustrated in FIG. 11. Thus, thethickness after etching of top silicon layer 404 defining electrodeinterconnects 112 may be about 5 μm in an embodiment in whichapproximately 5 μm of a 10 μm thick top silicon layer 404 is removed byDRIE etching. This is consistent with the 5 μm thick top silicon layer404 within flexible membrane 310 described above. Alternatively, thethickness of top silicon layer 404 within electrode interconnects 112may be about 3 μm in an embodiment in which approximately 5 μm of a 7 μmthick top silicon layer 404 is removed by DRIE etching. Accordingly, thethickness of top silicon layer 404 within electrode interconnects 112may be equal to the thickness of top silicon layer 404 within flexiblemembrane 310. However, the thickness of top silicon layer 404 withinflexible membrane 310 need not be the same as the thickness of topsilicon layer 404 defining electrode interconnects 112, but may insteadbe thinner or thicker. After DRIE etching, a buffered oxide etch havinghydrofluoric acid and a buffering agent is used to remove the islands902 without removing a substantial thickness of the buried oxide layer314, thereby revealing electrode surfaces 202, 204 and resulting in thestructure illustrated in FIG. 12. Next, a patterned positivephotoresist, e.g., having a thickness of about 12 to 15 μm, may befollowed by DRIE etching of previously etched areas of electrodeinterconnect 112 to form electrode traces, resulting in the structureillustrated in FIG. 13.

Referring to FIG. 14, a dielectric layer 312 is formed over top siliconlayer 404 in order to passivate the mesa structures 311 and electrodeinterconnect 112 and a backside dielectric layer 1402 is formed. Atomiclayer deposition, thermal oxidation, or chemical vapor deposition may beused to form a dielectric layer 312 over mesa structures 311 andelectrode interconnect 112, as well as within trench 1002, anddielectric layer 1402 on the back surface of the bulk silicon substrate704. Optionally, an insulating layer 1404 may be deposited overdielectric layer 312 using, for example, blanket atomic layerdeposition. In an embodiment, insulating layer 1404 includes Al₂O₃.Thus, the structure illustrated in FIG. 14, having dielectric layer 312and backside dielectric layer 1402, is reached.

Dielectric layer 312 may be formed from various materials, includingSiO₂, Al₂O₃, HfO₂, or SiN_(x). In accordance with embodiments of theinvention the gripping pressure generated by the array of electrostatictransfer heads 114 on the array of micro devices is proportional to thedielectric constant of dielectric layer 312, and thus, the choice ofdielectric material may be chosen to balance gripping pressure withmanufacturability. In an embodiment, dielectric layer 312 is formed fromAl₂O₃ having a thickness of about 5,000 angstroms and a dielectricconstant of about 9.

Referring to FIG. 15, a spring pre-release for forming cavity 212 iscreated around portions of electrode interconnect 112 in a multi-etchsequence. First, a patterned positive photoresist is applied andfollowed by RIE etching of dielectric layer 312 to buried oxide layer314. Optional insulating layer 1404 is not shown in FIG. 15, but in acase where insulating layer 1404 is included, RIE etching may also beused to etch through insulating layer 1404 to dielectric layer 312.Second, RIE etching of buried oxide layer 314 to bulk silicon substrate704 is performed, resulting in the structure illustrated in FIG. 15.

Referring to FIGS. 16-18, contact area 506 is exposed on topside plugarea 504 through a multi-etch sequence. First, the previously appliedpatterned positive photoresist can be removed before a new patternedpositive photoresist is applied and RIE etching of dielectric layer 312to top silicon layer 404 is performed, resulting in the structureillustrated in FIG. 16. Second, the patterned positive photoresist canbe removed before a second patterned positive photoresist is applied andDRIE etching of top silicon layer 404 to buried oxide layer 314 isperformed, resulting in the structure illustrated in FIG. 17. Third, RIEetching of buried oxide layer 314 to bulk silicon substrate 704 isperformed, resulting in the structure illustrated in FIG. 18, havingcontact area 506 exposed on a topside of bulk silicon substrate 704.

Referring to FIG. 19, topside contact 307 is formed through the openingand in electrical contact with contact area 506. The patterned positivephotoresist can be removed before a patterned negative lift-offphotoresist is applied and 500-1,000 angstroms TiW and 1,000-5,000angstroms Au is sputtered to create topside contacts 307, resulting inthe structure illustrated in FIG. 19.

Referring to FIGS. 20-21, a backside surface of bulk silicon substrate704 may be exposed through backside dielectric layer 1402 and backsideoxide layer 706. First, a patterned positive photoresist is applied andRIE etching of backside dielectric layer 1402 to backside oxide layer706 is performed, resulting in the structure illustrated in FIG. 20.Second, RIE etching of backside dielectric layer 1402 to bulk siliconsubstrate 704 is performed, resulting in the structure illustrated inFIG. 21.

Referring to FIG. 22, one or more contact pad 306 may be formed on abackside surface of bulk silicon substrate 704. About 500 to 1,000angstroms TiW and 1,000 to 5,000 angstroms Au may be sputtered to createcontact pads 306, resulting in the structure illustrated in FIG. 22.

Referring to FIG. 23, a gap 308 separating a plug 304 and a basesubstrate 214 of bulk silicon substrate 704 may be formed around contactpads 306. A patterned positive photoresist is applied and DRIE etchingof bulk silicon substrate 704 to buried oxide layer 314 is performed,resulting in the structure illustrated in FIG. 23. The arrangement ofplug 304 and base substrate 214 is described above. Plug 304 and basesubstrate 214 may be considered as portions of bulk silicon substrate704 that are defined during the formation of micro pick up array 104 andtherefore become individual features of micro pick up array 104.

Referring to FIG. 24, one or more cavities 212 may optionally be etchedin bulk silicon substrate 704 underneath the array of electrostatictransfer heads 114 such that the array of electrostatic transfers headsare deflectable into the one or more cavities 212. In an embodiment, aseparate cavity 212 is formed underneath each electrostatic transferhead 114. In an embodiment, a single cavity 212 is formed underneath thearray of silicon electrodes in electrical communication with theelectrode interconnects 112. In an embodiment, cavity 212 is formed witha timed release etch into the bulk silicon substrate 704 and undercutsthe electrode interconnect 112 and mesa structures. For example, etchingmay be performed with a fluorine based chemistry such as XeF₂ or SF₆.During etching, the backside of SOI stack 702 may be protected withdicing tape.

Following the optional formation of the one or more cavities 212, theSOI substrate may be diced, for example using laser dicing, to form oneor more micro pick up arrays 104 having compliant contactsinterconnected with electrostatic transfer heads 114 through electrodeinterconnects 112. Furthermore, the micro pick up array 104 may includeone or more contact pads 306 that electrically connect electrostatictransfer heads 114 with working circuitry or voltage sources 106, 206 oftransfer head assembly 102.

Referring again to FIG. 25, the system having micro pick up array 104physically and electrically coupled with transfer head assembly 102 maybe positioned over an array of micro devices 2510 on a carrier substrate2508. More specifically, the system may be moved relative to bothcarrier substrate 2508 and a receiving substrate while supplying anelectrostatic voltage to electrostatic transfer heads 114 as needed inorder to grip, transfer, and release micro devices 2510 from carriersubstrate 2508 to the receiving substrate. As an example, the receivingsubstrate may be, but is not limited to, a display substrate, a lightingsubstrate, a substrate with functional devices such as transistors orICs, or a substrate with metal redistribution lines. During movementbetween carrier substrate 2508 and the receiving substrate, the array ofmicro devices 2510 may be retained by the array of electrostatictransfer heads 114 using a persistent electrostatic gripping pressuremaintained by a transfer of voltage to electrostatic transfer heads 114.Alternatively, voltage application may be discontinued during movementbetween carrier substrate 2508 and the receiving substrate, and thearray of micro devices 2510 may still be retained against the array ofelectrostatic transfer heads 114 by non-electrostatic forces, such asvan der Waals forces. The array of micro devices 2510 may be releasedonto receiving substrate following transfer from carrier substrate 2508,for example, by discontinuing the voltage supply to electrostatictransfer heads 114.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications may be made thereto without departing fromthe broader spirit and scope of the invention as set forth in thefollowing claims. The specification and drawings are, accordingly, to beregarded in an illustrative sense rather than a restrictive sense.

What is claimed is:
 1. A micro pick up array comprising: a basesubstrate having a via; a flexible membrane over the via; a plug withinthe via and supported by the flexible membrane, wherein a gap separatesthe plug from the base substrate and the plug is moveable relative tothe base substrate; and an array of electrostatic transfer headselectrically coupled with the plug.
 2. The micro pick up array of claim1, wherein each electrostatic transfer head comprises: a mesa structurehaving an electrode surface, and a dielectric layer covering theelectrode surface.
 3. The micro pick up array of claim 2, furthercomprising an electrode interconnect electrically coupling the electrodesurface with the plug.
 4. The micro pick up array of claim 3, wherein atopside contact on the plug electrically couples the electrodeinterconnect with the plug.
 5. The micro pick up array of claim 4,wherein the topside contact contacts the plug over a contact area,wherein the contact area is coplanar with a topside plug area, andwherein the contact area is less than two-thirds of the topside plugarea.
 6. The micro pick up array of claim 4, further comprising acontact pad on the plug opposite the topside contact, wherein thecontact pad is electrically coupled with the topside contact through theplug.
 7. The micro pick up array of claim 6, wherein an electricalresistance across the plug between the contact pad and the topsidecontact is in a range between 1 and 100 kiloohms.
 8. The micro pick uparray of claim 3, wherein each electrostatic transfer head furthercomprises a second electrode surface adjacent the electrode surface,wherein the dielectric layer covers the second electrode surface, andwherein a second electrode interconnect electrically couples the secondelectrode surface with a second plug.
 9. The micro pick up array ofclaim 1, wherein each electrostatic transfer head is deflectable into acavity in the base substrate.
 10. The micro pick up array of claim 1,wherein the flexible membrane comprises a silicon layer.
 11. The micropick up array of claim 1, wherein the plug includes an axis orthogonalto the flexible membrane, and wherein the flexible membrane isconfigured to deflect such that the plug is moveable not more than 5 μmalong the axis relative to the base substrate.
 12. The micro pick uparray of claim 1, wherein a breakdown voltage of the gap is greater than100 volts at ambient pressure.
 13. The micro pick up array of claim 12,wherein the gap separates the plug from the base substrate by more than10 μm.
 14. A method of forming a micro pick up array comprising: etchinga top silicon layer of a silicon-on-insulator (SOI) stack to form anarray of electrodes; and etching through a bulk silicon substrate of theSOI stack to a buried oxide layer of the SOI stack to form a gapseparating a plug and a base substrate of the bulk silicon substrate,wherein the plug is moveable relative to the base substrate.
 15. Themethod of claim 14, further comprising etching the base substrate toform one or more cavities directly underneath the array of electrodessuch that one or more electrodes is deflectable into the one or morecavities.
 16. The method of claim 14, further comprising: etching thetop silicon layer to form an electrode interconnect; forming adielectric layer over the array of electrodes; and forming a topsidecontact on the bulk silicon substrate, wherein the topside contact iselectrically coupled with the array of electrodes through the electrodeinterconnect.
 17. The method of claim 16, wherein forming the dielectriclayer comprises thermal oxidation of the array of electrodes.
 18. Themethod of claim 16, wherein forming the dielectric layer comprisesblanket depositing the dielectric layer using atomic layer deposition.19. The method of claim 16, wherein forming the dielectric layercomprises depositing the dielectric layer using chemical vapordeposition.
 20. The method of claim 16, further comprising etchingthrough the dielectric layer, the electrode interconnect, and the buriedoxide layer to expose the plug of the bulk silicon substrate, whereinthe topside contact is formed on the plug.
 21. The method of claim 20,further comprising: etching through a backside oxide layer of the SOIstack to expose the plug of the bulk silicon substrate; and forming acontact pad on the plug of the bulk silicon substrate opposite thetopside contact, wherein the contact pad is electrically coupled withthe topside contact through the plug.
 22. A system comprising: atransfer head assembly including an operating voltage contact and aclamping voltage contact; and a micro pick up array including a basesubstrate, a compliant contact formed through the base substrate, and anarray of electrostatic transfer heads electrically coupled with thecompliant contact; wherein the operating voltage contact is alignablewith the compliant contact and the clamping voltage contact is alignablewith a backside of the micro pick up array opposite the array ofelectrostatic transfer heads on a frontside of the micro pick up array.23. The system of claim 22, wherein the micro pick up array comprises: avia in the base substrate; a flexible membrane over the via; a plugwithin the via and supported by the flexible membrane, wherein a gapseparates the plug from the base substrate and the plug is movablerelative to the base substrate; and wherein the array of electrostatictransfer heads are electrically coupled with the plug.
 24. The system ofclaim 22, wherein the compliant contact is alignable with the operatingvoltage contact such that when a clamping voltage is applied to theclamping voltage contact the micro pick up array is retained against thetransfer head assembly and the plug moves relative to the basesubstrate.
 25. The system of claim 22, wherein the transfer headassembly comprises a plurality of operating voltage contacts, and themicro pick up array comprises a plurality of compliant contacts, whereinthe plurality of operating voltage contacts are alignable with theplurality of compliant contacts.
 26. The micro pick up array of claim22, wherein each electrostatic transfer head comprises: a mesa structurehaving an electrode surface, and a dielectric layer covering theelectrode surface.